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Volume control [AVC?]

Discussion in 'Electronic Basics' started by Rodney Kelp, May 5, 2004.

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  1. Rodney Kelp

    Rodney Kelp Guest

    You could make a million if you design an automatic volume level controller
    for a televsion set. It would keep loud commercials low and low movies
    higher. In other words a steady volume level. Is that asking too much from
    today's technology? Even if you had to reroute the audio output of the TV
    to it and run the speakers off of it. I could make a mint just installing
    these things.
  2. Lord Garth

    Lord Garth Guest

    Doesn't (didn't) Magnavox market a TV with such a circuit already...
    Smart Sound isn't it?
  3. Bob Masta

    Bob Masta Guest

    Automatic volume controls are definitely non-trivial.
    The problem is attack and release times. If the
    gain is high for a soft part of the program, and there
    is suddenly an explosion, gunshot, etc, you want the
    AVC to catch it immediately, meaning it needs a fast
    attack time. But then you have problems with normal
    staccato things like close-up speech, since the AVC
    would treat each new syllable as a little gunshot. So
    you have a slow release time, to keep the AVC active
    at a more-or-less constant amount for the duration
    of the speech. But then after a gunshot, the gain
    stays reduced for a moment. The overall effect can
    be extremely annoying. Commercial units usually
    have user-adjustable attack and release times, but
    I'm not sure you can ever find a good compromise,
    except maybe by only applying a limited amount
    of AVC overall.

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
  4. Soeren

    Soeren Guest

    Hi Rodney,

    Most (if not all) commercials are compressed (dynamic range-wise), I
    cannot think of a single movie which is, so an AGC would have to react
    to the average sound pressure to have even the faintest influence.
    (The volume level does not change, at least not unless you fiddle with
    your remote ;)

    Some TV-sets have a preset for the volume level for each channel to
    avoid the differences when zapping, but that is an entirely different

    Only if they worked, so start by reading up on decompression.
  5. L. Fiar

    L. Fiar Guest

    You really think so?
    I already have a design, although it's main use is with SSB reception, to
    increase the volume on the quiet signals and reduce it on the loud ones. It
    uses a small IC, just a few passive components, and just 4 wires to connect
    it to the volume control.
    It's really not that complicated.
    It does depend upon how the volume is controlled within the TV. If it is
    controlled from the detector IC, then you need to take control of that
    control voltage.
    Otherwise, a small board can fit on the detector output, before the audio
    amp stages.
    Personally, I believe that the remote control makes such a device pointless
    on a TV.

  6. Such devices have been available (both externally for commercial
    level control) and internally (e.g. RCA 'sound logic.') One of my
    good friends purchased one of the audio levelling devices for his
    own TV recently (from one of those Fingerhut type catalogs.)

  7. My own design (works perfectly) adapts to the transient
    shape of the input signal. So, if you have a 'gunshot'
    and it determines that the predominant frequency components
    aren't 'too low', then the attack/release is quick.
    My design actually adapts to the shape of the transient
    signal, and doesn't just use the spectral shape.

    Another 'trick' is to make sure that the gain control is
    exponential/logarithmic instead of linear... Linear gain
    control tends to have problems (maintaining apparently
    consistent attack/decay over wide range.)

    One more 'trick' is to delay the audio signal enough that
    the envelope of the gain control doesn't cause troubles.
    What you do is to 'slow down' the gain control by a low
    pass filter, but delay the audio signal itself to accomodate
    the natural delay of the control signal. If done to extreme,
    then the signal will have an apparent 'real time' delay, but
    a little care will improve the sound quality.
  8. Peter

    Peter Guest

    Look for the old NE572 audio compander, still available. I build one as
    stereo compressor for my TV, works great, nothing revolutionnary.

  9. For that specific application (for the TV set thing), then
    the VERY old NE572 would be useful. I was also answering the
    issue of 'pumping' which is practically for a different application.
    (the AGC effects can be somewhat hidden by using some of the
    techniques that I had suggested.) For the TV set, a simple
    scheme that provides a (perhaps 10dB) gain control range, long
    time constant for decay (attack would best be approx 1-10ms),
    and pumping SHOULD be a non-issue.

    Other AGC devices might be more appropriate if the 572 is hard
    to find (e.g. other had to find analog devices components, or
    even thatcorp components.)

    Perhaps another approach might be to use a dual fet scheme (for
    stereo), if the ICs are too hard to find.

  10. Peter

    Peter Guest

    A friend of mine, just to do it differently, picked a LM13700 (dual OTA).
    After some experimentation, it's version seems to work fine.
    Analog Devices makes small 8-pin chips for microphone compression using only
    *one* resistor to adjust the compression ratio. I was very interested but
    couldn't get my hand on one :(

  11. Just another tidbit of info: there is a oft-used schemes using
    LDRs (light dependent resistors), but I forget the current tradename
    for them. Basically, they use the old light variable resistors with
    LED with varying intensity to change the resistance.

    The LDR thing is probably the highest easy quality. It is capable
    of full recording studio type quality, and is trivial complexity.

    The gilbert cell (or variants) including the various transconductance
    schemes can have extremely variable quality all the way from 'near
    perfection' on downwards to being distortion prone.

    Yet another scheme would be using the variable dynamic resistance
    of diodes, but there are balance issues (trying to avoid the gain
    change causing interference with the rest of the signal.)

    The variable gain thing is kind of a cool side-interest, but nowadays
    the DSP is the most fun.

  13. David Wood

    David Wood Guest

    Has anyone checked to see what relevant IC's Texas Instruments might
    have? They seem to be cutting edge on DSP (digital signal processing)
  14. Slightly diverted topic: if you have the 'energy', there is alot more
    flexibility in doing audio AGC in DSP. Perhaps the biggest issue is
    the input resolution (in the case of level compression), but the
    troublesome issues of dynamic attack/decay times are alot easier
    to implement through some simple nonlinear filter algorithms. In
    hardware, the equivalent array of sythesized nonlinar capacitors
    is messy.

    Probably all in all -- it is likely easier to do in analog hardware
    if there is no other reason for the DSP, but the DSP buys alot of
    flexibility. Also, when designing the algorithms a 32bit floating
    point (or bigger) can make it simpler than dealing with scaling for
    the integer schemes.

  15. I don't know if an audio leveler would have the effect you want.

    In the 1970's I designed a box to "eliminate" commercials. It crudely
    used the video "fade to black" as the trigger to signal a possible
    commercial. It was still up to the user to decide what to do about it.

    The Video interval test signal (VITS in RS-170A "analog") sends some
    of the following information: Closed captioning, XDS (call letters,
    other info) GCR (Ghost cancelling reference), network clock, and

    When local stations switch to their own commercials, the network clock
    disappears or it is momentarily interrupted. There are other changes
    in the VITS during commercials.

    Perhaps some of the VITS information - along with the video content -
    could be monitored to control the sound. Personally, I would like an
    indicator that tells me when the program I was watching comes back
    from commercial break.

    Frank Raffaeli
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